The present invention relates to a device for the measurement of the tension of blood gases and the resistance of the skin to the flow of such gases.
The invention described herein was made in the course of work under a grant or award from the Department of Health and Human Services.
Examples of single chamber transcutaneous probes are known in the art, such as shown in U.S. Pat. Nos. 4,005,700 and 4,220,158. Such probes may comprise a hollow body having a boundary wall which is placed on the skin, a single internal chamber in which gas diffusing through the skin contacted by the boundary wall can be collected, heating means operable to heat the area of skin and a single outlet from the collecting chamber through which the collected gas may be led away to an analysis instrument. Typically, the boundary wall may comprise a membrane of gas permeable plastic supported by a porous supporting member. The method employing such devices relies upon the diffusion of the blood gases from the capillary circulation beneath the skin to the surface of the skin. The measurement of the partial pressure of gases eliminated from the skin reflects its capillary gas tension. However, in order to more accurately reflect arterial gas tensions, the surface of the skin is heated to about 43.degree. C. so that blood flow to the capillaries is increased to such an extent that the gas tensions measured at the skin approximate arterial gas tensions. Such devices known in the prior art may be used for the measurement of the arterial blood content of certain gases, such as oxygen and carbon dioxide, without the need for arterial catheterization. Instruments for monitoring arterial oxygen and carbon dioxide are useful, for example, in neonatal care and during intensive medical care of adults.
One disadvantage of the prior art devices is that they cannot measure the resistance which the skin itself offers to the flow of gas from the blood through the tissues. Therefore, under any estimation of arterial blood gas tension using devices of the prior art, only changes in such blood gas tensions can be measured since it must be assumed that the skin resistance to gas flow remains constant. Therefore, absolute arterial blood gas tensions cannot be measured by devices known in the prior art since there was no way to measure skin resistance. Furthermore, the devices of the prior art may only be used under conditions in which it would be reasonable to assume that the skin resistance remains constant. Therefore, the measurements taken would only have meaning on a particular patient, taken on a particular area of skin, under conditions of temperature humidity, etc., for which it would be reasonable to assume that the skin resistance does not significantly change while measurements were being taken. However, the devices of the prior art may not be used when it is needed to compare arterial gas tensions between different patients or between different areas of skin on the same patient, since skin resistance varies in different parts of the body and from patient to patient. Also, under certain conditions, it has been found that skin resistance to diffusion of particular gases is time dependent. Therefore, in such cases the monitoring of a particular gas being eliminated from the skin tissues using devices of the prior art would not accurately reflect instantaneous arterial gas tensions.